1. Field of the Invention
The present invention relates to an opto-electronic integrated circuit used, for example, in optical fiber communication, for receiving light and converting it into electric signals.
2. Related Background Art
With the development of optical fiber communication, optical receivers used in the field are required to have high-speed operability. In order to satisfy this requirement of high-speed operability and to achieve miniaturization, there have been studied and experimentally produced various opto-electronic integrated circuits composed of a combination of a photodetector and transistors. A typical example of such opto-electronic integrated circuits is one including a light receiving circuit composed of a pin photodiode and hetero-junction bipolar transistors as described in "S. Chandrasekhar, et al.: IEEE Photon. Technol. Lett., Vol. 3, No. 9, 1991, pp. 823-825".
FIGS. 1 and 2 are structural drawings of the opto-electronic integrated circuit. FIG. 1 is a circuit structural drawing, and FIG. 2 is a table to indicate materials for forming the pin photodiode and hetero-junction bipolar transistors (hereinafter referred to as HBT). As shown, the opto-electronic integrated circuit is so arranged that the pin photodiode and HBT constituting a photodetector are formed, utilizing the hetero junction of InP and InGaAs, on a semi-insulating substrate made of an InP crystal. A chemical beam epitaxy process is used to form the hetero junction layer. The opto-electronic integrated circuit as so arranged is one for receiving light, having the characteristics of cut-off frequency (f.sub.T)=32 GHz and maximum oscillation frequency (f.sub.max)=28 GHz and being operable at 5 Gb/s for light of wavelength=1.5 .mu.m.
Further, S. Chandrasekhar et al. gave a report on an opto-electronic integrated circuit for receiving light, having almost the same structure as that of the above opto-electronic integrated circuit, formed using the MOMBE process, and being operable at 10 Gb/s for light of wavelength=1.53 .mu.m (S. Chandrasekhar, et al.: Electronics Letters, Vol. 28, 1992, pp. 466-468).
An equivalent input noise spectral density (N(f)) of opto-electronic integrated circuit can be expressed by the following formula (T. V. Muoi, IEEE/OSA Journal of Lightwave Technology, Vol. LT-2, No. 3, 1984, pp. 243-267: EQU N(f)=4kT/R.sub.F +2eI.sub.b +2eI.sub.c (2.pi.C.sub.T).sup.2 /g.sub.m.sup.2 +4kTr.sub.bb', (2.pi.C.sub.dsf).sup.2 f.sup.2 ( 1)
where N(f): equivalent input noise spectral density;
f: frequency; PA0 k: Boltzmann's constant; PA0 T: absolute temperature; PA0 R.sub.F : feedback resistance; PA0 e: elementary charge; PA0 I.sub.b : base current; PA0 I.sub.c : collector current; PA0 C.sub.T : total input capacitance; PA0 g.sub.m : current amplification factor of transistors; PA0 r.sub.bb' : base resistance; PA0 C.sub.dsf : a sum of photodetector capacitance, stray capacitance and feedback capacitance.
It is known that if first-stage amplification of a current generated in the photodetector with light input is performed by a common-emitter type amplifying circuit using HBT as described above, a noise component by the base current as expressed by the second term in above formula (1) becomes dominant in the equivalent input noise spectral density. Namely, the equivalent input noise spectral density can be approximately expressed by the following formula. EQU N(f).about.2eI.sub.b ( 2)
Accordingly, an amount of the noise component by the base current defines a limit to seeking high sensitivity.